One major field of application for this type of pumps is the injection of physiologically active fluid into a patient and/or the extraction of body fluid for diagnostic purposes. For this use the pumps are usually equipped with both a contact surface for attaching to a patient's skin and a cannula for the access to the patient's tissue or vessels for introducing an injection fluid or the removal of analysis fluid.
Injection devices are widely used in patient care but their size and complexity largely restricts their use to specialized facilities. Recently, ambulatory use of injection devices has been pioneered in diabetes care for the delivery of insulin. To achieve the necessary precision of delivery these injection devices typically use syringe pumps. The size of these devices is considerable, dictated mainly by the extended, longitudinal shape of a filled syringe with the drawn-out piston, and necessitates their wearing attached to e.g. a belt or underwear and they operate with connective barrels to a subcutaneously placed cannula leading to inconveniences and safety problems.
More recently, because of these drawbacks, infusion devices which can be attached directly to the skin, preferably without long barrels connecting to the subcutaneous delivery cannula are being developed. Due to the necessary reduction in size and weight the precise syringe-type pumps with sufficient volume of injection fluid are difficult to use attached directly to the skin. Therefore, alternative pump types with considerable drawbacks in precision and reliability of delivery under the highly variable environmental and physiologic conditions encountered during real-life operation have been incorporated, e.g. delivery from a reservoir with peristaltic pumps, with piston pumps using valves, or by squeezing a flexible container. Syringe pumps for such applications have barrels with a wide diameter in order to avoid an extended longitudinal footprint, but this solution has disadvantages necessitating high driving forces to inject against a considerable tissue back-pressure and most importantly because of the risk of air bubbles entering and occluding the injection cannula due to almost unavoidable relatively large dead-volumes, and unintended, relatively large bolus injections due to stick-slip effects.
An appealing solution to reduce the footprint of syringe-type pumps with appropriately narrow barrels is to use an arcuate barrel as described e.g. by M. P. Loeb and A. M. Olson in U.S. Pat. No. 4,525,164, filed in 1981. In spite of the attractiveness of this concept for precise patch-type infusion pumps, conversion to safe and cost-effective medical products is not evident, due to considerable practical difficulties in manufacturing such toroidal syringe pumps with the necessary performance at adequate costs. Obviously, such products have to use for the production of the arcuate barrel plastics-technologies with inherent significant tolerance margins because of differences in shrinkage. Since e.g. the mandrel of the injection molding tool has to be removed by a rotary motion and differences in shrinkage of the proximal and distal wall of the torus takes place thereafter, the resulting deviation from an ideal circular shape cannot be easily corrected by adapting the tooling accordingly. The almost unavoidable deviation from an ideal circular shape for the manufactured torus and the deformations under high forces necessary to overcome high tissue back-pressure leads to problems in achieving a sufficient sealing with the piston at reasonable friction and avoiding sticking due to the not perfect fit between the arc of the barrel and the rotary movement of the rigid arcuate driving rod of the piston. These difficulties get even more pronounced at low barrel diameters required for precise syringe-type pumps e.g. for insulin delivery. Despite several more recent descriptions of arcuate syringe pumps, e.g. by R. Paul Mounce et al. in WO 2008/024812 A2 or by O. Yodfad et al. WO 2008/139458 A2 this problem has not been adequately addressed and no practical solutions are obvious from the descriptions or figures.
The aim of the present invention is to provide an arcuate syringe type pump which avoids the disadvantages of the state of the art devices.